Traditionally, the methods for purifying phosphoric acid include an extraction method, e.g. the method disclosed in U.S. Pat. No. 6,251,351 extracting phosphoric acid produced by the wet process with N,N-dialkyl-α-aminoacid in order to increase the purity of phosphoric acid; a precipitation method, e.g. the method diclosed in U.S. Pat. No. 4,986,970 using dithio carbonic acid-O-ester to separate heavy metals by precipitation; and an ion-exchange method, e.g. the method disclosed in U.S. Pat. No. 5,006,319 using a strong alkaline anionic exchange resin to remove metal ions from phosphoric acid. However, the quality of phosphoric acid produced by these methods is far inferior than the quality of phosphoric acid produced by the dry process. Thus, these methods are rarely used in the industries. U.S. Pat. No. 4,495,165 discloses a purification method for phosphoric acid produced by the wet process including absorbing the phosphoric acid produced by the wet process with porous sawdust, heating the phosphoric acid absorbed in the sawdust to form polyphosphoric acid and/or phosphoric acid esters, and using water to hydrate the polyphosphoric acid and/or phosphoric esters in the porous sawdust into phosphoric acid. According to this process, about 90% to 95% of the impurities in the phosphoric acid produced by the wet process remain in the porous sawdust. Thus, a partial purification effect can be achieved. However, said purification method is a batch process and has an extremely high cost. Furthermore, the purity of phosphoric acid produced by said method is still far inferior than that of the dry process. For example, the iron content in the phosphoric acid produced by the dry process is less than 10 ppm, and the iron content in phosphoric acid purified by said method is higher than 100 ppm.